Water trees occur when an organic polymeric insulation material such as a polyolefin is subjected to an electrical field over a long period of time in an environment where water exists. They are called trees because they look like branched trees. In actuality, however, water trees are fine passages or voids which are visibly white when filled with water, but become invisible when dry. For purposes of observation, samples containing water trees must be boiled in water or dyed.
In contrast, electrical trees, which are conventionally called trees, are visibly black because they grow as a result of electrical discharges that carbonize the insulation material, leaving carbon therein. Chemical trees, on the other hand, occur in the insulation material of a cable, as for example, a copper cable, placed in an environment where a reactive gas, such as hydrogen sulfide, infiltrates to the surface of the copper conductor and forms copper sulfides and copper oxides thereon which grow into needle-like crystals. Thus, chemical trees appear black or reddish-brown in color and occur even when there is no electrical stress.
It is known that water trees grow from points in electrical insulation material, produced, for example, from polyolefin compositions, in which there are voids or extraneous matter such as metal. This is particularly true in the case of an insulated electrical cable which is subjected to high voltages while in a water environment, e.g. under water. Nevertheless, it is difficult to keep extraneous matter out of the insulation and to prevent voids from occuring, in spite of continued efforts to improve production processes.
In order to cope with this difficulty, the problem of water trees, it has been proposed to add to the polyolefin polymer compositions, small quantities of a salt of a strong electroylte as disclosed in published Japanese Patent No. 52-24938. Some still favor the belief that the presence of metal ions is a cause of water trees.
In Japanese laid-open Patent No. 52-145799, which discloses the utilization of large quantities of inorganic fillers as a counter measure for the degradation by water trees, there is disclosed an example in which an inorganic filler is used in combination with diethylene glycol. It should be noted, however, that the reason for using diethylene glycol is not explained in this patent. Also, an example without diethylene glycol, as set forth in this patent, gave a better result, in preventing water trees.
Another suppressant for degradation of an insulation material subjected to high voltage under water is disclosed in Japanese laid-open Patent No. 53-3700. According to one example of this patent, polyethylene glycol distearate, in minute amounts, is used as a suppressant. In view of the fact that this suppressant is merely as effective as metal salts of stearic acid, and that fatty acids, metal salts of fatty acids, and esters of fatty acids are referred to as suppressants, it is considered that the suppressant must be a fatty acid derivative having hydrophilic radicals, which are not specified in the specification. It should be noted that this patent makes no reference to polyethylene glycol.
Japanese laid-open Patent No. 54-60346 claims that a compound of less than 24 carbon atoms having in its molecule both a carboxylic acid moiety or a metal salt thereof, which have a great affinity for extraneous matters such as metals, and a double bond that chemically combines with polyolefins, is extremely effective as a modifier for crosslinked polyethylene compositions.
Among many proposals as counter measures for ordinary trees is the addition of a voltage stabilizer or alcohol. For instance, the effectiveness of a monohydric higher alcohol having a carbon number of from 6 to 24 is described in Japanese laid-open Patent No. 53-16746. The technology of this patent is disadvantageous in that the effectiveness of the alcohols does not last long. Such low molecular weight alcohols easily move among polyolefin molecules in the insulation and ooze out, as the conductor gets hot, to the surface of the cable from whence they evaporate or drop away.
Also, there is described in Japanese laid-open Patents Nos. 49-119937, 52-40547, 52-40553 and 52-40554, the addition of a polyhydric alcohol to tree resistant electrical insulating materials which contain, as an essential component, a voltage stabilizer such as ferrocene, dihydronaphthalene, dialkoxynaphthalene, phosphorane and para-nitroaniline halide. According to these patents, the addition of a polyhydric alcohol is claimed to effect an improvement in the dielectric strength.
It should be noted, however, that the polyhydric alcohol is merely described as a component of the same category as dispersants, surfactants and unsaturated polymers. Further, the polyhydric alcohols are effective only to such a degree that a slight difference can barely be recognized in the examples. In addition, polyhydric alcohols disclosed in these patents vary widely in molecular weight. The problem dealt with in these patents is not water trees which grow in the presence of water, but ordinary electrical trees which grow in insulation subjected to high voltages, as previously explained.
In the above noted Japanese laid-open Patents, polyhydric alcohols are described, secondarily, as water tree suppressants. Such polyhydric alcohols are represented by ethylene glycol, propylene glycol and butanediol, which are listed collectively together with a number of alicyclic polyhydroxyl compounds and aromatic polyhydroxyl compounds.
It should be noted that only polyethylene glycol is effective and polypropylene glycol is not, as established in Common Assignee's U.S. Application, identified below. This suggests that polyethylene glycol works in a manner different from that of polyhydric alcohols, used secondarily, according to the prior art.
The invention of U.S. Application Ser. No. 178,406, filed Aug. 15, 1980, now U.S. Pat. No. 4,305,849 granted Dec. 15, 1981, the disclosure of which is incorporated by reference, is based on a finding that water tree-induced degradation can be prevented by simply adding a small quantity of high-molecular weight polyethylene glycol to a polyolefin, without adding a voltage stabilizer, a carboxylic acid or metal salt thereof. This is quite different from the prior art wherein polyhydric compounds such as ethylene glycol or diethylene glycol were used, secondarily, in combination with voltage stabilizers; or polyethylene glycol end-blocked with stearic acid was used; as water tree suppressants.